Worldwide, H. pylori is the leading cause of gastritis, peptic ulceration and gastric malignancy. In developing countries, such as Chile, 60-80% of children are infected with H. pylori, but they rarely develop ulceration. In adults, chronic H. pylori infection, which is T helper 1 (Thl)-mediated, predisposes to peptic ulceration and gastric adenocarcinoma. Since large-scale medical eradication is impractical, an effective vaccine is highly desirable. Development of such a vaccine requires elucidation of the mechanism(s) by which H. pylori induces inflammation, particularly in children, the principal target population for vaccination. Accordingly, we hypothesize that: (1) H. pylori infection in children (<18 yrs) induces a local T regulatory (Tr) response (IL-10 and TGF-( production) that down-regulates an underlying Thl response and reduces the frequency of peptic ulceration. (2) In a mouse model that recapitulates human H. pylori infection, H. pylori urease (or other antigens) drives an IL-10/TGF-( dominated Tr response in young animals but a predominant Thl response in adult animals. (3) Delivery of H. pylori urease and either IL-10 or FoxP3 in a novel poliovirus vaccine vector to young mice protects the animals against H. pylori. These hypotheses will be tested with three specific aims: 1) Determine whether H. pylori infection in children and adolescents induces gastric T cell-derived IL-10 and TGF-( (a Tr response), in contrast to IFN-y and IL-2 (a Thl response) characteristic of H. pylori-infected adults. 2) Determine whether the Tr vs Thl dichotomy in H. pylori-infected children vs adults occurs in H. pylori-infected mice in order to define the molecular mechanisms of protection. 3) Determine whether vaccination of young mice with a novel poliovirus vector (replicons) incorporating the gene for the B subunit of H. pylori urease plus IL-10 or FoxP3 (the Tr cell transcription factor) protects mice against H. pylori infection.